Here as promised is the how-to replace your transfer case output shaft seal (the one that goes to your drive-shaft). Note that this was performed on a 2000 v6 manual extended cab tundra.
To get ready to do this, you need some tools and parts.
The seal was P/N 90311-41012 and cost about $11.50 from the stealership.
Be very particular to actually see what part you are getting, because the first seal I got was the input transfercase seal (which goes between the tranny and the tcase.
The thing is, they will both fit the shaft, but the outer diameter is not the same. Note that both Advance and Auto Zone had this input tcase seal, so I really could only get the output seal from the dealership.
Next you need some tools. You will need a 14mm wrench to pull off the drive-shaft, a 12mm wrench is you decide to pull off the tcase skid plate, a 24mm socket (I think) to change the fluid, a 30 mm socket to pull off the tcase output nut (preferably impact), Impact wrench or V.long breaker bar, small punch, large hammer (blacksmiths), seal puller, possibly a two jaw puller, and a propane torch. Note that I didn't use all of these, but some of them would have helped.
First you need to pull off the drive-shaft. Here I used a combination of box end wrenches and special high torque wrenches that don't strip the nuts.
Here is also where I didn't use the propane torch, but it would have helped. The nuts are held on with Toyota thread locker, and you CAN bust them loose without too much trouble, but heat would make it a lot easier.
After pulling off the four nuts and washers, you can just set the shaft to one side, I didn't need to pull the entire thing out. (NOTE - to rotate the shaft and break the nuts, you will need to turn the 4wd on and off repeatedly)
After the shaft is moved to the side, you can see the 30 mm nut that is punched to keep it from rotating. First you need to use the punch and hammer/mallet to push the indentation back out. I used a round punch and it worked okay, my Chilton's has the guy using a flat punch, but it's really small so you may want to get it from a specialty store, Auto Zone and Advance didn't have a flat punch (only round). I really wasn't able to make this look nice, but I got it most of the way there.
Now, this is really a make or break part, as I could not get the nut off with just a two ft cheater bar. I was able to borrow a friend's impact and it came off without really any fuss. I didn't have to really hammer on it, it came off quickly, but it's just not something that a 2ft cheater bar will do.
After the nut is off, things get real easy. You can drain the t-case before hand. I didn't.. You pull the flange off, I started to use a puller, but very quickly it came loose and I just pulled it off... It really wasn't even worth putting it on, considering how easily it came off...
Take a good look at the flange and make sure it's not damaged. Note that there is an internal seal right behind the large nut. Check to make sure it's not damaged.
Now get out your seal puller and negotiate the seal out... It took almost no effort to get it out, and that's it your at the halfway point!!!
Now is a good time to look at the seal that you are pulling out... You will notice an outer dust flap, an outer sealing edge and an inner sealing edge... On mine, there was some grime in between the inner and outer sealing edges, and that caused the inner edge to wear away. This is what caused the leak, the seal was no longer small enough.
Get out your new seal and you will see that it should be 59 mm in outer diameter.
With some grease inside the seal and outside, I was able to push the seal in BY HAND!!! Took no "Negotiating."
After this, clean the flange and slip it back over the splines of the tcase output. My nut was marked in blue marking paint and was easy to torque back to what it was originally (Chilton's calls for 87 ft-lbs, I'm sure you can do this, but as long as you punch it, it's not going anywhere). I re-punched the original tab back in.
Filled the T-case with some 75-W90 mobile 1 fully synthetic. Check for any major leaks. Reattach the drive-shaft to the flange. I didn't do this (cause I forgot), but apply some locking compound to the flange studs and tighten down the nuts... And you are done!